1. Field of the Invention
One embodiment of the invention disclosed herein relates to power receiving devices and contactless power feeding systems.
2. Description of the Related Art
Various electronic devices have spread, and a wide variety of products are in the marketplace. In recent years, portable electronic devices such as mobile phones and digital video cameras are used very commonly. Further, electric propulsion vehicles, such as electric vehicles, which are supplied with power based on electricity, are showing up in the market.
In such mobile phones, digital video cameras, or electric propulsion vehicles, batteries serving as power storage units (also referred to as storage batteries) are incorporated. Currently, such a battery is usually charged by being placed in direct contact with a household AC power source, which is one of power feeding units. In a structure which has no battery or which does not use power stored in a battery, a device is operated by power fed directly from a household AC power source via a wire or the like.
On the other hand, methods by which batteries are charged without contact or power is fed to loads without contact have been researched and developed. Typical methods are an electromagnetic coupling method (also referred to as an electromagnetic induction method) (see Reference 1), a radio wave method (also referred to as a microwave method), and a resonance method (also referred to as a resonant method) (see References 2 to 4).
As described in References 2 to 4, in a contactless power feeding technique using a resonance method, a device that receives power (hereinafter referred to as a power receiving device) and a device that feeds power (hereinafter referred to as a power transmitting device) each have a resonant coil. Further, in each of the power receiving device and the power transmitting device, an electromagnetic coupling coil is provided. Feeding power from a power source to the resonant coil in the power transmitting device and feeding power from the resonant coil to a load in the power receiving device are conducted by the electromagnetic coupling coils.
The resonant coil of the power transmitting device and the resonant coil of the power receiving device are adjusted to exhibit resonance (LC resonance) at the same frequency ƒ0.
When the resonant coil of the power transmitting device and the resonant coil of the power receiving device face each other, they exhibit a resonance (resonant) phenomenon, and thereby efficient power transfer is achieved even when the distance between the resonant coils is large (see Reference 5).